In recent years, Homogeneous-Charge Compression Ignition (HCCI) combustion in which a gasoline fuel mixed with air combusts by self-ignition inside a fully-compressed combustion chamber has attracted attention. Since the HCCI combustion is a form in which the mixture gas combusts simultaneously without flame propagation, the combusting speed of the mixture gas is faster than SI combustion (jump-spark-ignition combustion) which is adopted by normal gasoline engines. Therefore, it is said that the HCCI combustion is very advantageous in terms of thermal efficiency. However, it is necessary to solve various problems of automobile engines which require an improvement in thermal efficiency, and engines which operate by a suitable HCCI combustion have not yet been put into practical use. That is, although the engines mounted on automobiles vary largely in the operating state and the environmental condition, the HCCI combustion has a problem in which a combustion start timing of the mixture gas (timing at which the mixture gas self-ignites) changes largely by external factors, such as temperature, and also has a problem in which control during a transition operation in which load changes suddenly is difficult.
Thus, it is proposed that, without combusting all of the mixture gas by self-ignition, a portion of the mixture gas is combusted by the spark ignition using a spark plug. That is, a portion of the mixture gas is forcibly combusted by flame propagation (SI combustion) triggered by the spark ignition, and the remaining mixture gas is combusted by self-ignition (CI combustion). Hereinafter, such combustion is referred to as “SPCCI (SPark Controlled Compression-Ignition) combustion.”
JP2009-108778A is known as one example of an engine adopting a concept similar to the SPCCI combustion. This engine carries out flame-propagation combustion by the spark ignition of a stratified mixture gas formed around a spark plug (ignition plug) by a supplementary fuel injection and then carries out a main fuel injection into a combustion chamber which reaches a high temperature by the effect of the flame-propagation combustion (flame) to cause the fuel injected by the main fuel injection to combust by self-ignition.
The CI combustion of the SPCCI combustion takes place when an in-cylinder temperature (temperature inside a cylinder) reaches an ignition temperature of mixture gas which is defined by the composition of the mixture gas. Fuel efficiency can be maximized if the CI combustion occurs by the in-cylinder temperature reaching the ignition temperature near a compression top dead center. The in-cylinder temperature increases with an increase in an in-cylinder pressure (pressure inside the cylinder). The in-cylinder pressure on the compression stroke when the SPCCI combustion is carried out is increased by compression work of a piston and combustion energy of the SI combustion. Therefore, if the flame propagation of the SI combustion is not stable, increasing amounts of the in-cylinder pressure and the in-cylinder temperature resulting from the SI combustion decreases, and it becomes difficult to raise the in-cylinder temperature to the ignition temperature. If the in-cylinder temperature does not fully rise to the ignition temperature, more of the mixture gas combust by flame propagation with a long combustion period because of a reduction in the amount of the mixture gas which carries out the CI combustion, or the CI combustion takes place when the piston descends considerably, and as a result, fuel efficiency decreases. Thus, in order to cause the stable CI combustion to maximize fuel efficiency, it is important to stabilize the flame propagation of SI combustion.
In addition, the SPCCI combustion also requires improvements in fuel efficiency and exhaust performance, by turning the combustion chamber into an air-fuel ratio (A/F) lean environment in which an air-fuel ratio which is a ratio of air to fuel inside the cylinder becomes higher than a stoichiometric air-fuel ratio. However, in such an A/F lean environment, it becomes difficult to stabilize the flame propagation of the SI combustion.